1. Field of the Invention
The present invention relates to a sheet processing apparatus and a sheet processing system formed by connecting a plurality of sheet processing apparatuses. In particular, the present invention relates to correcting for positional errors of sheets of paper being input into and output out of the sheet processing apparatuses.
2. Description of the Related Art
Conventionally, there has been known a technique of correcting lateral shift or skew of a sheet so as to improve sheet processing accuracy in a sheet processing apparatus.
For example, in a sheet processing apparatus disclosed in Japanese Patent Laid-Open Publication No. 2007-055748, when hole punching is to be performed, a “lateral shift amount” indicative of an amount of sheet shift in a sheet width direction orthogonal to a sheet conveying direction is detected before execution of the hole punching. Then, “lateral shift correction” is performed in which the lateral shift amount is corrected and compensated for, whereby the accuracy of positioning punched holes is improved.
Further, in a sheet processing apparatus disclosed in U.S. Pat. No. 7,520,497, a “skew amount” indicative of the amount of angular shift of the leading edge of a sheet is detected before execution of hole punching, and “skew correction” is performed in which the skew amount is corrected and compensated for, whereby the accuracy of positioning of punched holes is improved.
As is apparent from the above description, hole punching performed by a sheet processing apparatus requires correction time for correcting the lateral shift or skew of a sheet and time for punching holes in the sheet. The required correction time depends on the lateral shift amount or skew amount of a sheet, and as the lateral shift amount or skew amount is larger, the correction time is longer. For this reason, processing steps are generally configured to attempt to process sheets efficiently even when the position correction time is at a maximum.
In a known sheet processing system, a plurality of sheet processing apparatuses are connected in series in a sheet conveying direction so as to perform various kinds of sheet processing such as stacking, folding, hole-punching, collating, stapling, etc., which tends to increase the total length of the sheet processing system. A longer sheet conveying passage is more likely to cause positional errors of a sheet. Further, the number of connection sections between processing apparatuses increases, so that positional errors are more likely to occur when the sheet passes between the apparatuses or through the connection sections therebetween.
To improve the processing accuracy of the apparatus and protect it against occurrence of a lateral shift or skew of a sheet, there has been proposed a system in which each of a plurality of connected sheet processing apparatuses is provided with not only a lateral shift detecting mechanism and a skew detecting mechanism, but also a lateral shift correcting mechanism and a skew correcting mechanism. Such a system is configured such that the lateral shift amount and the skew amount are detected and then lateral shift correction and skew correction are performed in each apparatus incorporating the above-mentioned mechanisms, so as to prevent degradation of sheet processing accuracy.
However, when a lateral shift or skew of a sheet occurs on a conveying passage in one of the apparatuses or in a connection section between two of the apparatuses, extra time is required for correcting the lateral shift or skew in an apparatus downstream of the conveying passage or connection section, which causes an increase in sheet processing time.
Let it be assumed that a stacker 400 is disposed on the upstream side and a finisher 100 is disposed on the downstream side, as shown in plan view in FIGS. 23A and 23C. Assuming that in a case where the stacker 400 on the upstream side is displaced laterally (or in a transverse direction) with respect to the sheet conveying direction toward the finisher 100 as shown in FIG. 23A, if a sheet P is subjected to lateral shift correction in the stacker 400 and is then conveyed out therefrom with the center of the sheet being positioned to the center of the stacker 400 in the transverse direction, the sheet conveyed into the finisher 100 on the downstream side is laterally shifted as shown in FIG. 23B. On the other hand, in a case where the stacker 400 on the upstream side is disposed in a state angularly displaced with respect to the conveying direction while the finisher 100 is straight, for example, as shown in FIG. 23C, a gap on the bottom of FIG. 23C representing the front side of the sheet processing system is created between the stacker 400 and the finisher 100. If a sheet discharged from the stacker 400 without being skewed with respect to the stacker 400 is conveyed into the finisher 100 in the above-mentioned state of the stacker 400 and the finisher 100, the sheet is skewed in the finisher 100, as shown in FIG. 23D. If the skew has the leading edge thereof slanted toward the front of the sheet processing system (downward as viewed in FIG. 23D), this may be referred to as a “frontwardly skewed state”.
When the number of apparatuses connected in the system increases, even if each of the apparatuses is provided with a detecting mechanism for detecting a lateral shift or skew of a sheet and a correcting mechanism for correcting the lateral shift or skew of the sheet, lateral shift and skew can be caused when the sheet passes between the apparatuses. Further, with the increase in the number of the apparatuses, the number of connection sections inevitably increases, which is more likely to cause a lateral shift or skew of a sheet.
On the other hand, when lateral shift correction or skew correction is not properly performed in each of the apparatuses, there is a risk of accumulation of lateral shift or skew of a sheet before the sheet reaches the next sheet processing apparatus downstream thereof. When sheet processing is performed by the downstream sheet processing apparatus, sheet position correction time corresponding to the accumulated amount of lateral shift or skew of the sheet is needed for the sheet processing. Therefore, it is necessary to secure sufficient correction time for performing the lateral shift correction or skew correction in the downstream apparatus. For this reason, it is necessary to perform processing with a sufficient sheet feed interval, and hence there is a risk of the productivity of the system being reduced. However, an attempt to shorten the correction time so as to prevent reduced productivity leads to degraded processing accuracy.
Further, depending on the direction of shift of a sheet or that of displacement between adjacent apparatuses, the direction of a correction to be performed by each apparatus can be opposite to that of a correction previously performed, and hence it is possible that a correction in an upstream apparatus is negated by a positional error further downstream.